Al-containing III-V group compound semiconductors have been, and will be, of significant importance. One reason is that their large band gap energy permits light emission at shorter wavelengths, or UV band, which cannot be achieved by other semiconductors. AlN, for example, has a band energy gap of 5 to 6 eV. In comparison, the band gap of GaN, a compound known to have relatively large band gap, is at most about 3.5 eV.
Al-containing III-V compound semiconductors can be used to make various light sources that operate at different wavelength bands, such as high intensity light-emitting diodes used in various display devices, lasers used for reading/writing CDs and DVDs, and lasers for optical communication. These semiconductor light sources are essential in the modern IT society.
Another reason that Al-containing III-V group compound semiconductors are so important also comes from their large band gap energy: they can be used to make harsh environment-resistant semiconductors that are less susceptible to malfunctions when exposed radiation. The term “harsh environment” as used herein includes an environment surrounding a radiation source such as nuclear reactor, and an environment in which flying objects and artificial satellites are exposed to cosmic rays when flying at high altitudes.
This characteristic band gap is brought about by the presence of aluminum (Al). Accordingly, Al-containing III-V group compound semiconductors, such as AlN, AlGaN, and AlGaInP, that contain different amounts of aluminum, a group III element, are needed to serve as the core of the above-described semiconductor light sources and harsh environment-resistant semiconductors.
In general, light-emitting parts of the semiconductor light sources and operative parts of the harsh environment-resistant semiconductors are formed by depositing a thin layer of a few microns or less in thickness. This can be done by using known techniques such as liquid phase epitaxy (LPE), molecular beam epitaxy (MBE), and metalorganic vapor phase epitaxy (MOVPE).
These deposition processes, however, require the use of a “substrate” that is 100 micron or greater in thickness. Such a substrate is difficult to fabricate by any of the LPE, MBE, and MOVPE techniques: the MBE and MOVPE, though suitable for depositing a thin layer of a few microns or thinner, take substantial amounts of time to form a thick layer of 100 microns or thicker and are therefore not practical.
On the other hand, LPE is suitable for growing a relatively thick layer of up to about 100 microns but the technique is not appropriate for growing a layer over a large area or for the mass production of layers due to the nature of the growing technique. Specifically, the LPE technique, which involves melting a metal into a liquid phase to form a layer, has drawbacks including high energy required in the melting process and insufficient “wetting” of the substrate with the melt. These drawbacks make the technique less practical.
Another technique, known as hydride vapor phase epitaxy (HVPE), is used for depositing a 100-micron or thicker layer. The HVPE technique involves sending a gas current of a halogenated product of Ga or In so that it can react with a hydrogenated product of a group V element to form a compound semiconductor. This technique is suitable for making thicker layers and is sometimes referred to as “halide vapor phase epitaxy.”
The HVPE technique uses a quartz reaction tube and is of a hot wall type, in which not only the crystal growth area but also the surrounding quartz reaction tube is heated to a high temperature. In comparison, the MOVPE technique is of a cold wall type, in which only the substrate crystal is heated, but not the surrounding quartz reaction tube. Also, the above-mentioned MBE technique uses an ultra high vacuum chamber and no quartz is included in the reaction system.
A quartz reaction tube hot wall type, the HVPE technique has an advantage of particularly fast growth. For this reason, the technique has been used in the production of high sensitivity photosensors that require thick layers and power devices that require a thick, high-quality crystal (in particular, power source devices using GaAs). It is also used in the production of above-described substrates, in particular, GaN substrates. These applications are described in, for example, Japanese Patent Laid-Open Publication No. Hei 10-215000 entitled “Process for growing crystal of gallium nitride-containing compound semiconductor (TOYODA GOSEI K. K. et al.)” and Japanese Patent Laid-Open Publication No. Hei 10-316498 entitled “Epitaxial wafer and production method thereof (SUMITOMO DENKO K. K.).”
When an attempt is made to employ the MBE or MOVPE technique to grow, via heteroepitaxy, an Al-containing semiconductor, such as AlGaN, on a GaN substrate fabricated by the HVPE, the difference in the lattice constant or in the coefficient of thermal expansion between the substrate and the deposited layer causes cracks to form in the Al-containing crystal formed on the GaN layer. This has kept many application devices from being realized.
To address this problem, it was suggested to use the HVPE technique to make the substrate for the Al-containing III-V group compound semiconductor. This approach, however, had a significant problem: the halogenated product of aluminum (e.g., AlCl and AlBr) vigorously reacts with the quartz-made reaction vessel due to the nature of aluminum as a group III element. This reaction results in contamination of the compound semiconductor with Si from the quartz reaction vessel. Moreover, the quarts vessel may itself be damaged. For these reasons, the Al-based HVPE technique has been considered unsuitable for the epitaxial growth of semiconductors despite the high growth rate, and thus, the high productivity offered by the technique.